1. Field of the Invention
The present invention relates to offset cancelling circuitry for correcting deviations, or offsets, in reference level among a plurality of video signals each being associated with a particular color and, more particularly, to offset cancelling circuitry advantageously applicable to a digital electronic still camera or similar video equipment.
2. Description of the Related Art
A digital electronic still camera or similar video equipment capable of converting an image to electric signals by, for example, solid state image sensors and storing them in a memory card is a recent achievement in the imaging art. The video signals to be applied from the image sensors to processing circuitry may be implemented as analog red (R), green (G) and blue (B) color signals each being derived from a particular color filter provided on an image pick-up cell array. The R, G and B signals are dot sequentially inputted to respective channels of the processing circuitry, subjected to level adjustment and other various kinds of signal processing including clamping, and then transformed to digital data by an analog-to-digital converter (ADC). Since circuitry with a highly accurate ADC is expensive, it has been customary to sequentially convert the color signals to digital data by a single line having a multiplexer and an ADC. The digital data are subjected to various kinds of digital processing including gamma correction and then written into a memory card. The gamma correction corrects a difference between the input characteristic of an image input system and the display characteristic of a cathode ray tube or similar display device, as well known in the art. The gamma correction is executed by the equipment which shoots a scene. A gamma curve representative of a correction characteristic is such that the amplification factor or gamma gain is high in a range where the input signal of the equipment is low, i.e., in a low luminance range.
However, the problem with the conventional circuitry built in digital electronic still camera is that there occurs deviations or offsets between the reference level and the R, G and B signals due to irregularities among the elements constituting the R, G and B channels of the circuitry. Then, the R, G and B signals sharing a single multiplexer and a single ADC are digitized by the ADC without having their offsets in reference level corrected and then the digitized signals undergo gamma correction. As a result, the offsets among the color signals are also amplified and superposed on the expected color signals as undesirable color deviation data. The color deviation data is especially serious when it involves data lying in a comparatively low luminance range where the gamma gain is high. As a result, an image with a low luminance cannot be accurately reproduced in respect of colors. To eliminate this problem, offset adjustment is effected channel by channel such that the R, G and B signals match one another with respect to the reference level. However, the conventional offset adjustment should be extremely accurate and should not change over a long period of time. Moreover, such offset adjustment is done by hand.